Trunked cellular communication systems are known. Such systems are typically divided over geographical areas into substantially contiguous service coverage areas, each served from a base site. Mobile communication units passing through a service coverage area served by a base site typically transmit service requests to the base sites, which requests are answered with a resource allocation dedicated by the base site for use by the requesting communication unit. Such a system is typically referred to as a frequency division multiple access system (FDMA).
Communication resources used by a base site and allocated to the communication system may consist of a frequency spectrum divided for use by the system into a number of communication resources. Each communication resource may consist of a pair of frequencies. The pair of frequencies may be used by a communication unit to transmit and receive information.
Communication resource use by a base site may also be divided by function with at least one resource, in some systems, reserved for the two-way transmission of control information between the base site and communication units passing through the service coverage area of the base site. Other resources may be reserved for use by communication units for communication transactions following allocation of a resource for such use following request and allocation by the base site.
Since a limited number of communication resources may exist within a frequency spectrum the reuse of communication resources is necessary within the geographic area served by the communication system. Reuse, on the other hand, is often limited by mutual interference between communication units, and base sites, operating on the same frequency in the same system.
Reuse of a communication resource within a communication system is limited by a number of factors. Chief among the factors, specifically in a communication system having an omnidirectional antenna structure, is distance between reusing transmitters and the power of the transmitted signal. Where transmitting power is fixed within communication units by design then the remaining factor determining reuse is distance. The distance between reusing communication units is often specified in terms of the ratio of the distance (D) between the centers of nearest, neighboring co-channel cells to the radius (R) of a cell. The reuse ratio, D/R, specifies the proximity of the closest reusing base site.
Another factor affecting reuse is the transmitted power of a communication signal. As the power of the transmitted signal is reduced the proximity between reusing base sites becomes smaller and smaller. One patent teaching of such an approach is that of Cunningham et at. (U.S. Pat. No. 4,144,496). Cunningham teaches of a method of subdividing a large cell into a number of smaller cells. Communication resources assigned to the large cell are divided among the smaller cells. Directional antennas are also used in Cunningham to isolate reusing base sites in an effort to reduce co-channel interference.
The use of directional antennas was also taught in Graziano (U.S. Pat. No. 4,128,740) as a method of reducing the reuse factor. Directional antennas were used in Graziano in combination with a resource assignment algorithm to produce a co-channel reuse ratio of 4.
Graziano and Cunningham rely upon fixed assignment methods and directional antennas to reduce the incidence of interference. The incidence of interference, on the other hand, is not a constant phenomenon. Reuse of communication resources, as is known, may occur whenever the incidence of interference is below a threshold relative to the desired signal, based on measured signal parameters.
Measured parameters determining reuse factors for a system may be based upon a determination of signal to interference (S/I) for a desired transmitter in the presence of an interferer and where interference is a combination of thermal and co-channel interfering signals. (An alternate term, C/I, or carrier to interference, is more frequently used in determining reuse factors.) A minimum reuse factor, as is known for analog radios, may be achieved whenever the measured signal from the desired transmitter (C/I ratio) is 17 dB above measured interference.
In the past reuse factors could be determined by placing a desired transmitter at the edge of the service coverage area of a home cell and measuring the distance to an interferer in cell diameters to achieve the 17 dB difference in signal strengths. Under past advances in technology, however, reuse factors are calculated based upon computer simulations.
Advances (described in Cunningham), resulting in a reduced reuse factors, have been the development of directional antennas and selective assignment of frequencies. Past developments (as in Graziano) have included cells divided into as many as six sectors with a 60 degree sector antenna providing coverage within each sector. The selective assignment of resources then precluded reassignment of adjacent channel resources within adjacent sectors. Use of the 60 degree sector antennas, and the selective assignment of frequencies, as mentioned, have resulted in reuse factors as low as four.
Digital communication systems such as code division multiple access (CDMA) are also known. Such systems may operate in a frequency hopping mode under which transmitting and receiving units transceive on a given communication resource for only a short period before indexing to a new resource in a list of assigned resources. CDMA communication systems, as is known, have a much greater resistance to interference than FDMA or TDMA communication units. The greater resistance to interference of CDMA systems may be attributable to the limited transmission time on any given resource and the spectrum limited scope of interference sources. Other sources of resistance to interference within CDMA systems arise from the broad range of coding techniques for the CDMA signal and the variable signal content of coded speech. Digital communication systems, as is known, are capable of proper operation in C/I environments of at least 9 dB 90% of the time or C/I environments of at least 17 dB 50% of the time.
Because of increasing competition for a limited frequency spectrum a need exists to allow operation of competing communication systems on identical frequencies without mutual interference and with a minimum reuse factor. Such a communication system must have an increased sensitivity to a desired signal or a reduced susceptibility to interference.